Document the functionality of copy_from_user_nmi() to avoid further
confusion. Fix the typo in the existing comment while at it.

Requested-by: Borislav Petkov <b...@alien8.de>
Signed-off-by: Thomas Gleixner <t...@linutronix.de>
---
V2: New patch
---
 arch/x86/lib/usercopy.c |   22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

--- a/arch/x86/lib/usercopy.c
+++ b/arch/x86/lib/usercopy.c
@@ -9,9 +9,23 @@
 
 #include <asm/tlbflush.h>
 
-/*
- * We rely on the nested NMI work to allow atomic faults from the NMI path; the
- * nested NMI paths are careful to preserve CR2.
+/**
+ * copy_from_user_nmi - NMI safe copy from user
+ * @to:                Pointer to the destination buffer
+ * @from:      Pointer to a user space address of the current task
+ * @n:         Number of bytes to copy
+ *
+ * Returns: The number of not copied bytes. 0 is success, i.e. all bytes copied
+ *
+ * Contrary to other copy_from_user() variants this function can be called
+ * from NMI context. Despite the name it is not restricted to be called
+ * from NMI context. It is safe to be called from any other context as
+ * well. It disables pagefaults across the copy which means a fault will
+ * abort the copy.
+ *
+ * For NMI context invocations this relies on the nested NMI work to allow
+ * atomic faults from the NMI path; the nested NMI paths are careful to
+ * preserve CR2.
  */
 unsigned long
 copy_from_user_nmi(void *to, const void __user *from, unsigned long n)
@@ -27,7 +41,7 @@ copy_from_user_nmi(void *to, const void
        /*
         * Even though this function is typically called from NMI/IRQ context
         * disable pagefaults so that its behaviour is consistent even when
-        * called form other contexts.
+        * called from other contexts.
         */
        pagefault_disable();
        ret = __copy_from_user_inatomic(to, from, n);

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